Switch-mode power supplies are often used to provide a regulated DC voltage in computers, television receivers, battery chargers, etc. Switch-mode power supplies are typically smaller, lighter, and more efficient than linear power supplies, which are also commonly used to provide regulated DC voltages. A typical circuit for a switch-mode power supply includes a capacitor, an inductor (e.g., a primary winding of a transformer) and a switch, e.g., a transistor, such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). These components are normally disposed on a single side of a circuit board, such as printed circuit board (PCB), and, in one configuration, are connected in series using, for example, electrically conducting traces to form a power loop. Another example of a power loop is a circuit including a secondary winding of a transformer, a rectifier (e.g., a diode or a MOSFET), and a capacitor connected in series.
When current flows in the power loop, the current flows from the capacitor, through the inductor, through the switch, and back to the capacitor. The switch is opened and closed at a high rate to produce a pulsed current flow in the loop. Moreover, additional circuitry, e.g., a control circuit for controlling the switch, is frequently disposed on the same side of the circuit board and is connected to the power loop using, for example, electrically conducting traces. The control circuit usually operates at substantially lower current levels than does the power loop and includes a number of sensitive inputs. Moreover, the control circuit is often located in close proximity to the power loop of the switch-mode power supply. In many instances, the control circuit and the power loop are connected to the same ground conductor on the circuit board. This frequently causes problems because the pulsed current flow in the power loop produces relatively high levels of noise in the ground conductor that frequently gets passed to the control circuit via the sensitive inputs.
Another problem is that the power loop acts as an antenna for transmitting the noise. Increasing the area enclosed by the power loop increases the noise transmission. Disposing the control circuit in proximity to the power loop on the same side of the circuit board can result in the control circuit being exposed to increased noise levels that interfere with the operation of the control circuit.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for switch-mode power supplies that are configured for protecting sensitive control circuitry from noise generated by the power loop within the switch-mode power supply.